That each planet and satellite did form originally a ring we still have evidence in the wonderful and beautiful rings of Saturn, which, however, in all probability will eventually form spherical satellites like the rest. Thus then our Earth was originally a part of the Sun, to which again it is destined one day to return. M. Plateau has shown experimentally that by rotating a globe of oil in a mixture of water and spirit having the same density this process may be actually repeated in miniature.
This brilliant, and yet simple, hypothesis is consistent with, and explains many other circumstances connected with the position, magnitude, and movements of the Planets and their satellites.
The Planets, for instance, lie more or less in the same plane, they revolve round the Sun and rotate on their own axis in the same direction—a series of coincidences which cannot be accidental, and for which the theory would account. Again the rate of cooling would of course follow the size; a small body cools more rapidly than a large one. The Moon is cold and rigid; the Earth is solid at the surface, but intensely hot within; Jupiter and Saturn, which are immensely larger, still retain much of their original heat, and have a much lower density than the Earth; and astronomers tell us on other grounds that the Sun itself is still contracting, and that to this the maintenance of its temperature is due.
Although, therefore, the Nebular Theory cannot be said to have been absolutely proved, it has certainly been brought to a high state of probability, and is, in its main features, generally accepted by astronomers.
The question has often been asked whether any of the heavenly bodies are inhabited, and as yet it is impossible to give any certain answer. It seems à priori probable that the millions of suns which we see as stars must have satellites, and that some at least of them may be inhabited. So far as our own system is concerned the Sun is of course too hot to serve as a dwelling-place for any beings with bodies such as ours. The same may be said of Mercury, which is at times probably ten times as hot as our tropics. The outer planets appear to be still in a state of vapour. The Moon has no air or water.
Mars is in a condition which most nearly resembles ours. All, however, that can be said is that, so far as we can see, the existence of living beings on Mars is not impossible.
COMETS
The Sun, Moon, and Stars, glorious and wonderful as they are, though regarded with great interest, and in some cases worshipped as deities, excited the imagination of our ancestors less than might have been expected, and even now attract comparatively little attention, from the fact that they are always with us. Comets, on the other hand, both as rare and occasional visitors, from their large size and rapid changes, were regarded in ancient times with dread and with amazement.
Some Comets revolve round the Sun in ellipses, but many, if not the majority, are visitors indeed, for having once passed round the Sun they pass away again into space, never to return.
The appearance which is generally regarded as characteristic of a Comet is that of a head with a central nucleus and a long tail. Many, however, of the smaller ones possess no tail, and in fact Comets present almost innumerable differences. Moreover the same Comet changes rapidly, so that when they return, they are identified not in any way by their appearance, but by the path they pursue.